The present invention relates to the field of lithography and in particular, to a method of patterning a membrane.
In lithography, a photoresist is formed on a substrate and an image of a desired pattern is introduced into the photoresist. In order for the pattern to have the desired accuracy, it is desirable for the photoresist to have uniform thickness.
Typically, the substrate is rigid. U.S. Pat. No. 6,033,135 to An et al. describes inverting and spinning a rigid substrate. In some applications (such as a SCALPEL mask), the substrate is a thin, non-rigid membrane mask. U.S. Pat. No. 5,260,151 to Berger et al. describes a SCALPEL membrane mask.
A problem with a membrane mask is that the membrane mask has a tendency to bow under the weight of the photoresist, resulting in a photoresist of non-uniform thickness. FIG. 1 illustrates a membrane mask 10 supported by a support structure 20. As illustrated, when photoresist 30 is coated on top of the membrane mask 10, the weight of the photoresist 30 makes the membrane mask 10 sag in areas 12 between the struts 22 of the support structure 20. As a result, the photoresist 30 is thicker in area 32, between the struts 22 of the support structure 20, than in area 34. The resist non-uniformities caused by the sagging causes streaking regions in the final product.
Further, current spin coat application requires that the resist is coated on the membrane as a thick substrate arrangement and then spun. The role of the spin motion during spin coating is two-fold: first, to spread the film by centrifugal force and second, to dry the film as the solvent evaporates from the surface. The thickness non-homogeneity caused by the application of the photoresist, illustrated in FIG. 1, is worsened by the spin motion during spin coating.
FIGS. 2-3 also illustrate masks made by conventional spin coat processes mentioned above. FIG. 2 illustrates an example of spincast thin film non-uniformity caused by membrane sag. The zoomed portion of FIG. 2 shows how film thickness modulation over a mask membrane changes from a center of the mask to edges of the mask. In particular, the zoomed portion of FIG. 2 shows a xe2x80x9cnarrowerxe2x80x9d window 100, a xe2x80x9cwiderxe2x80x9d window 102, and a resist streak 104.
FIG. 3 illustrates the effect of changing drying characteristics in a spin casting process. The recipe used for FIG. 3 was the same as for FIG. 2, except a cover of the spinner was on during the spread, coat, and dry stages. FIG. 3 illustrates resists streaks 104 as well as a stretched resist image 106 of the grid. It is evident from FIG. 3 that the film acquires a shape and then xe2x80x9cstretchesxe2x80x9d during the spread and dry spin stage. It is also evident from FIG. 3 that the thickness modulation pattern extends beyond the mask membrane area.
The present invention is directed to a process for depositing an energy-sensitive composition, such as a photoresist or other energy-definable material on a non-rigid membrane substrate, where the non-rigid membrane substrate is flipped as part of a spin-casting process.
The present invention solves the problem described above with conventional processes by applying a photoresist on a membrane substrate, spinning the membrane substrate right side up to wet the membrane substrate with the photoresist, then further spinning with the membrane substrate inverted so that the photoresist-coated membrane substrate is upside down. The upside down membrane substrate does not bow in response to the weight of the photoresist.
The photoresist is further spun substantially upside down in a xe2x80x9cclosed-gapxe2x80x9d environment. A closed-gap environment usually has a gap of 2 mm or less between the photoresist and a spinning chuck. The closed gap inhibits turbulence and limits evaporation of the photoresist during spinning because the small gap inhibits air convection.
The photoresist wets the membrane substrate surface, which ensures that the applied photoresist does not drip off the membrane substrate. Most photoresists will adequately wet the membrane substrate surface, but wetting may depend on the membrane substrate material and particular photoresist.
The present invention can be applied to the coating of SCALPEL mask membranes with a chemically amplified photoresist. The present invention permits greater flexibility in mask processing not attainable with conventional resist coating processing techniques. Furthermore, the present invention can be used in the process of any non-rigid membrane, such as those encountered in the MEMs fabrication process, X-ray lithography masks and stencil masks (such as used for ion-beam projection lithography and PREVAIL scattering masks).
Experimental data illustrates that substantially more uniform films can be made with the method of the present invention.